1. Field of the Invention
The present invention relates to thin film resistors and, more particularly, to a thin film resistor and method of forming the resistor on spaced-apart conductive pads.
2. Description of the Related Art
A thin film resistor is a structure that is formed from a conducting resistive material. As with conventionally-formed discrete resistors, thin film resistors are formed to provide a predefined resistance to the flow of current through the semiconductor structure.
FIGS. 1A-1B to 5A-5B show a series of views that illustrate a prior-art method 100 of forming a thin film resistor. FIGS. 1A-5A show a series of plan views, while FIGS. 1B-5B show a series of corresponding cross-sectional views. As shown in FIGS. 1A-1B, method 100 begins with a conventionally-formed layer of insulation material 110, and continues with the deposition of a thin layer of resistor material 112, such as a layer of silicon carbide chrome (SiCCr) or nickel chrome (NiCr), on insulation layer 110.
After resistor material 112 has been deposited, a mask 114 is formed and patterned on resistor material 112. Following this, as shown in FIGS. 2A-2B, the exposed areas of resistor material 112 are etched to form a thin-film resistor 116 from resistor material 112. Once the etch has been completed, mask 114 is removed.
Next, as shown in FIGS. 3A-3B, a first layer of conductive material 120, such as titanium tungsten (TiW), is formed on insulation layer 110 and resistor 116. After this, a second layer of conductive material 122, such as aluminum, is formed on the first layer of conductive material 120.
Once the second layer of conductive material 122 has been formed, a mask 124 is formed and patterned on the second layer of conductive material 122. Following this, as shown in FIGS. 4A-4B, the exposed areas of the second layer of conductive material 122 are anisotropically etched, followed by the anisotropic etching of a portion of the exposed areas of the first layer of conductive material 120 to form an opening 126.
Since the first layer of conductive material 120 is partially removed with an anisotropic (dry) etch, the first layer of conductive material 120 must be sufficiently thick to ensure that the anisotropic etch does not etch through the first layer of conductive material 120 and erode or remove any portion of thin-film resistor 116 that lies underneath.
After the anisotropic etch has been completed, the exposed areas of the first layer of conductive material 120 are isotropically (wet) etched as shown in FIGS. 5A-5B with an etchant that has a high selectivity to the material of resistor 116 until the first layer of conductive material 120 has been removed from the top surface of resistor 116. Following this, mask 124 is removed.
One problem with method 100 is that the first layer of conductive material 120, which has to be sufficiently thick to avoid damage to thin-film resistor 116, must be wet etched for a relatively long period of time (over etched) even though it has been partially etched during the anisotropic etch to ensure that the first layer of conductive material 120 has been completely removed.
If the first layer of conductive material 120 is not completely removed, stringers 128 can remain which, in turn, can short out the resistor. Stringers 128 are tiny strips of the first layer of conductive material 120 which can remain after the first layer of conductive material 120 has been removed from the top surface of resistor 116.
However, the longer the first layer of conductive material 120 is exposed to the isotropic etchant to ensure the removal of stringers 128, the greater the length L1 (the width of the opening shown in FIG. 5B). The length L1 defines the length of resistor 116 which, in turn, defines (in part) the resistance provided by resistor 116. As a result, it becomes difficult to control the resistance provided by resistor 116.
Thus, there is a need for a thin film resistor and method of forming the resistor that reduces variations in the length of the resistor.